“Alicorn” originally referred to a unicorn’s horn. The name in Italian was “li corne”, from Latin “cornu”; the Arabs borrowed it as “al-licorn”; the English got it from the Arabs as “alicorn”. There are many English words with an Arabic “the” tacked on at the front (algebra and alcohol, for example), but this is the only one I know of where the Arabs accidentally borrowed someone else’s “the” as well.
 
Wikipedia:  
“The horn itself and the substance it was made of was called alicorn, and it was believed that the horn holds magical and medicinal properties. The Danish physician Ole Worm determined in 1638 that the alleged alicorns were the tusks of narwhals. Such beliefs were examined wittily and at length in 1646 by Sir Thomas Browne in his Pseudodoxia Epidemica.
 
“False alicorn powder, made from the tusks of narwhals or horns of various animals, has been sold in Europe for medicinal purposes as late as 1741. The alicorn was thought to cure many diseases and have the ability to detect poisons, and many physicians would make “cures” and sell them. Cups were made from alicorn for kings and given as a gift; these were usually made of ivory or walrus ivory. Entire horns were very precious in the Middle Ages and were often really the tusks of narwhals.”
 
(Note that narwhal horns turn blue in the presence of bases, and most poisons in the Middle Ages and early modernity were bases, so there was some truth to the belief that an alicorn would turn blue in the presence of poisons – like Discord in the presence of respiratory diseases. There wasn’t any truth to the belief that an alicorn would render poisons harmless, though.)
 
As for when “alicorn” started to mean “winged unicorn”, I don’t know; for what it’s worth, I first encountered that use of the word in Friendship is Magic itself, which I think will go down in history as the codifier of an awful lot of magical horse tropes.

@Background Pony #4E1D  
Oh my god!  
History! I’ve learned so much!  

 
((I could’ve replied in a more professional tone, but where’s the fun in that? Thanks anyway.))

@HJSDGCE  
Piers Anthony used “alicorn” to refer to a winged unicorn in one of his Xanth novels in the ’80s. He says he got the term from an advertisement for fantasy miniatures. Someone posted on Usenet about 20 years ago that, in the mid ‘70s, a friend of hers came up with the term “alacorn” (derived as illustrated above) because he hated the term “pegacorn”; I suspect that term, being used more often in speech than in writing, got confused with the already-existing term “alicorn”, and that’s how the spelling changed.

@Kazapsky  
Really?  
Wow… How come I couldn’t find it in the Internet before?

@HJSDGCE
 
“Alicorn” as a term for winged unicorns existed before the basic concept for FiM crossed Lauren Faust’s mind.

@Background Pony #28F0  
They actually added ‘alicorn’ in dictionaries?  
We just made that phrase!

In the show, it’s spelled “Alicorn”. Dictionaries also back this up.

I thought those were some kind of horn magic glands or something at first.

Eh, just posted it to see what kind of discussion it would spawn. I’m really not all that nitpicky about the naming thing. That goes for a lot of things.

One tiny mistake. The latin for garlic is Allium.
 
The most likely derivation of alicorn is
 
ali- “another,other”
 
cornus “horn”
 
alicorn. “other horn”
 
Given that European experience of horned animals was generally that they had two, one on each side of the head, a horn in the centre of the forehead would be “another”.
 
Anyway, even if that’s all wrong, ali- still has no relation to garlic.

@Background Pony  
Wherein the Alicorn himself describes it as a triple-corruption, such that “the alicorn” translates to “the the the horn.”

*but now it’s

@Background Pony  
Or, your know, language evolves, no need to stick with the old forever as long as there is no harm in it, then again “gay” used to mean happy but know it’s homosexual…

Actually, the term for a winged unicorn may well have started out as “alacorn”. Per Kay Shapero (click the subject line from ‘Kay S.’ dated 6/16/93):  

I can easily see “alacorn” being misspelled as “alicorn”, especially due to the previous existence of that term in connection with unicorns, and eventually making its way to Piers Anthony, who used it in Bearing an Hourglass.

sorry, shit connection

Nope.
 
ali- is an acceptable prefix.
 
 
“Alicorn” came from “a licorne” which means “from the unicorn.” Referred to their horn, came to present meaning in 1980s.

Nope.
 
ali- is an acceptable prefix.
 
 
“Alicorn” came from “a licorne” which means “from the unicorn.” Referred to their horn, came to present meaning in 1980s.

Nope.
 
ali- is an acceptable prefix.
 
 
“Alicorn” came from “a licorne” which means “from the unicorn.” Referred to their horn, came to present meaning in 1980s.

I’m still surprised no one has done a Muhammad Ali-corn.

so we’ve just been spelling it wrong, ok.